Free Spins Retrigger
A retrigger extends a running free spins round when the trigger lands again, and the rules should state how many extra spins arrive and any cap.
How retriggers work
A retrigger extends a Free Spins round that is already running. Where the triggering symbols land again during the round, the game adds spins to the remaining count. The added number is stated in the rules, and it either matches the first award or follows a smaller schedule.
Games handle retriggers in three broad ways. Some allow them without limit, some cap the total number of spins in one round, and some exclude retriggers entirely. All three designs are legitimate, and the only failure is a rule book that does not say which of them applies.
The retrigger condition is not always the opening trigger. A game that opens its round on 3 Scatter Symbols can demand the same 3 again, accept 2 during the round, or move the trigger onto a different symbol. The paytable states the working rule, usually in the same section that describes the round.
A retrigger changes what a round is worth, because an extendable round carries more spins on average than its stated count. For that reason the presence or absence of a retrigger belongs in any fair reading of a bonus, alongside the multiplier behaviour and the symbol changes inside the round.
The lines to find in the rules are short. They are the retrigger condition, the number of spins added and the cap, where one exists. A game that states all three has documented the feature properly.